Automobile engine spark plug electrodes erode during use because of spark erosion as well as because of the corrosive effect of tetraethyl lead and other additives in the fuel or engine oil. The erosion increases the spark gap, requiring increasingly greater voltage, which, in turn, accelerates the rate of erosion.
Typically, the electrodes of automobile spark plugs are fabricated from nickel-chromium alloys comprising in the range of about 80-98% nickel and 2-15% chromium, optionally alloyed with small amounts (e.g., 2-5%) of manganese and/or iron. In order to reduce the rate of electrode erosion, it is known to fabricate the electrodes from noble metals, for instance, an alloy of 60% gold and 40% palladium, or an alloy of 96% platinum and 4% tungsten containing dispersed thorium oxide. However, these alternative alloys are expensive and the 60% gold-40% palladium alloy is adversely affected by lead-containing compounds, such as tetraethyl lead.
In avoiding or reducing pollutants discharged to the atmosphere in automobile engine exhaust gas, it is advantageous to improve the effectiveness and efficiency of the spark ignition of fuel. One means for accomplishing this is to employ spark plug center electrodes having a diameter of about 0.01-05 inches. This permits plug designs which cause less masking of the ignition arc and can result in a more efficient firing of the fuel. Although it is possible to fabricate nickel-chromium alloy center electrodes of such relatively small diameter, commercial practice has been to fabricate such electrodes with diameters of about 0.1-0.12 inches, in order to avoid the rapid rate of erosion which occurs with nickel-chromium alloy electrodes of smaller diameters.
Equally as important as minimizing spark erosion of the electrodes is avoiding fouling of the plug, which can result in misfiring, with resulting lower efficiency and increased emissions. To minimize fouling, good spark plug design calls for the nose of the plug to project into the cylinder. However, this causes the tip of the plug to run hotter and can result in faster erosion.
The center electrode of a jet engine igniter is usually made of tungsten or an alloy comprising predominantly tungsten, whereas the ground electrode is often a nickel alloy, such as Inconel. Because the severe operating conditions in a jet engine (about 1800.degree. F. or higher) favor erosion, however, igniters are normally employed only during take-off, landing and during adverse atmospheric conditions conducive to flame-out. Improved erosion resistance would permit longer or continuous operation of a jet igniter, thus improving reliability and safety.
Ruthenium, iridium, platinum and alloys or mixtures thereof have been suggested in the prior art for use as spark plug and jet igniter center electrodes because of their arc erosion resistance and their high melting points and hardness. However, in all of the prior art constructions, the quantity of the precious metal employed per electrode is so high that the price of the spark plug becomes excessive and uneconomical. In addition, commercially pure ruthenium cannot, by currently known commercial technology, be satisfactorily reduced in size by hot or cold working, and the difficulty in fabricating crack-free articles made of ruthenium has deterred the use of it as a center or ground electrode.
A number of methods have been described to avoid such problems. In the manufacture of alloys for use as switch contacts, U.S. Pat. No. 3,278,280 describes a liquid phase sintering method by which a metal powder mixture of ruthenium, gold and palladium is heated to a sufficiently high temperature to liquify and distribute the gold-palladium phase. In another method, the patent describes a process of compacting ruthenium powder and then infiltrating the compact with a molten gold-palladium alloy. The workability of the resulting duplex alloy is said to be superior to that of pure ruthenium. U.S. Pat. No. 3,498,763 describes an improvement wherein the above-mentioned gold-palladium phase is replaced with a matrix of copper, or an alloy of copper with nickel and/or palladium, and states that the workability of the product also produced by a liquid phase sintering, is improved to the point that it "may readily be drawn to wire".
U.S. Pat. No. 3,868,530 describes the use of gold, platinum, palladium, rhodium, rhenium, ruthenium, and tungsten as spark plug center electrodes. A precious metal tip in the form of a fine wire (typically 0.010 to 0.030 inches in diameter) is inserted into a recess in a nickel rod or tube and the tube or rod is swagged to reduce its diameter somewhat to effectively lock the fine wire precious metal into place. The criterion given for the selection of the precious metal alloy is that it be such that it can be drawn into fine wire form and that it will resist sparking erosion.
The methods described in U.S. Pat. Nos. 3,278,280 and 3,498,763 are less than satisfactory. The liquid phase sintering method they describe is an expensive, difficult process and the resulting products are difficult to draw to the very fine diameters desired for a spark plug or jet igniter center electrode application. As a practical matter, metals such as ruthenium are not commercially available in fine wire form as referred to in U.S. Pat. No. 3,868,530, primarily because of the extreme difficulty in fabricating crack-free wrought wire from these metals by the methods taught in the patent. In addition, the handling and insertion of small diameter (0.005 to 0.02 inches diameter) short length inserts is a tedious and difficult operation.
Spark plug electrodes may also be made by the methods disclosed in U.S. Pat. Nos. 3,977,841 and 3,957,451, wherein ruthenium powder is mixed with a prealloyed powder of cobalt, nickel, chromium, tungsten, and silicon and then pressed and sintered in the liquid phase at temperatures of about 2150.degree. F. to 2250.degree. F. Silicon is known to form with ruthenium a eutectic melting at a low temperature. This should affect adversely the arc erosion properties of the composite.
Composite spark plug electrodes have heretofore been of the solid base metal core-type in which a center metal is covered by a plating or cladding of a more spark erosion resistant material. Typical of such electrodes are those disclosed in U.S. Pat. Nos. 2,783,409, 3,119,944, 3,356,882, and 3,857,145.
Composite wire for welding rods and the like has been produced by filling a metal tube with powder and drawing the composite to the appropriate size. Such methods are disclosed in U.S. Pat. Nos. 2,888,740, 3,391,444, 3,533,152, 3,824,097 and 3,922,769 and generally relate to the application of hard facings by means of such composite welding rods and the making of so-called "superalloys".